Electronically commutated motors, known as EC motors, are often used for driving fans. These drives generally consist of a permanent magnet synchronous motor (PMSM) with integrated power and signaling electronics. They are often external rotor motors.
Such electric motors can be operated with a single phase or three-phase line AC voltage by initially rectifying the line AC voltage into a DC link voltage, which is then converted via a controlled inverter into a motor operation voltage for supplying and commutating the motor.
To achieve an optimally uniform, constant motor torque with minimum ripple and thereby favorable noise behavior (particularly for fan drives), the DC link voltage should be as constant a DC voltage as possible. Until now the rectified, strongly pulsating DC voltage was smoothed by means of at least one smoothing capacitor and optionally an additional filter choke. For this purpose, the smoothing capacitor must actually have a rather high capacitance (e.g. several hundred μF), so that electrolytic capacitors are typically used. In practical use, the latter have certain disadvantages, however, particularly a large overall size and a short lifetime.
Therefore there is an increasing tendency today, either to do without smoothing capacitors at all, or at least to forgo electrolytic capacitors, in which case longer-lived film capacitors with a lower capacitance (only up to a few μF) are used. This is called a “slim DC link,” wherein a decoupling of the line and motor sides by means of storage components such as capacitors and coils (reactors) is forgone completely or at least to a large extent. This means that a slim DC link contains either no DC link reactance or only a minimum amount.
Problems appear with this “slim DC link” technology, especially in case of a supply from a single phase line, since the rectified DC voltage pulsates very strongly at a frequency of, for example, 100 Hz with a voltage curve corresponding to the sinusoidal line AC voltage. If an EC motor (PMSM) were supplied directly with such a strongly pulsating DC voltage, then no motor current could be impressed into the motor windings below a certain limit voltage. The consequence would be a torque drop.
Several possibilities for keeping the torque as constant as possible despite the rippling DC link voltage are known.
Thus, the torque can be kept constant by regulating the current. The disadvantage in this case is that the motor currents have to be detected to accomplish this, and that an excessively fast current regulation can induce a system consisting of a system inductance and a DC link capacitor to oscillate. The result would be an even more pulsating DC link voltage.
The use of controlled rectifiers in combination with the inverter of the EC motor is also known (cf. for example the literature citation “Low-loss inverter without DC link capacitor (German: Verlustarmer Umrichter ohne Zwischenkreis-Kondensator)” by B. Piepenbreier and L. Sack in “Elektronik 2006 No. 1”, pp. 61-67). This is concretely implemented by special matrix converters with controlled rectifiers. The disadvantage of this in principle well-functioning arrangement is the extra cost due to the additional controllable semiconductors, which becomes noticeable particularly in large series production.
The document JP 57177292 A relates to a torque control system for a collectorless DC motor, wherein a step-up chopper as well as a step-down chopper are arranged in a DC link in order to make the pulsating DC voltage more uniform. To accomplish this, the voltage is reduced by the peak voltage at certain times and by zero at other times. This document is only concerned with torque control and not with the technology of a “slim DC link,” particularly since a smoothing capacitor and an additional inductor are provided in the DC link, so that one cannot speak of a “slim DC link.”
The same applies to document U.S. Pat. No. 4,855,652 A, according to which a step-up chopper in the DC link is also provided with an additional inductor.
Finally, the publication US 2002/0089303 A1 also describes a driver circuit for an electronically commutated motor with a step-up chopper for a rectified pulsating voltage, wherein the step-up chopper, called an “energy return stage”, there consists of a capacitor, a switching element and a series circuit of an inductor and a diode. Here as well, the special technology of a “slim DC link” is not addressed.
The present invention is based on the problem of optimizing an electronically commutated electric motor (EC motor), especially one with a “slim DC link,” in a technically favorable manner and by simple and inexpensively realizable means.
This is achieved according to the invention by a method in accordance with this invention. A control system suitable for applying the method is also the subject matter of this invention. Advantageous embodiments of the invention are also specified in the description below.
According to the invention the pulsating DC voltage, i.e. a voltage that is not constant over time, that was initially generated by rectifying the line AC voltage is dynamically increased with respect to its instantaneous values using a step-up chopper in such a manner that the resulting DC link voltage with reduced ripple always lies above a defined limit voltage over time. The voltage that is sufficient for the respective electric motor to be always able to impress a motor current onto the windings via the inverter over the entire phase curve is specified as the limit voltage. With a suitable design of the step-up chopper and its controller, a nearly constant DC voltage can advantageously be generated from the strongly pulsating DC voltage. For this purpose, the pulsating DC voltage is connected to an inductor at a pulse duty ratio regulated on the basis of the respective current DC link voltage, and added up via a freewheeling diode and a DC link capacitor downstream of the inductor. A simple and inexpensive film capacitor of the order of only a few μF is sufficient for the DC link capacitor.
Since the inductance required for the functioning of the step-up chopper actually contradicts the “philosophy of a slim DC link,” it is further provided according to the invention that stray inductances of the motor winding heads, which are necessarily present in any case, are used as the inductor for the step-up chopper. In this manner an additional inductor can be eliminated in the slim DC link in contrast to the above-explained prior art according to JP 57177292 A, U.S. Pat. No. 4,855,652 A and US 2002/0089303 A1. To be able to use the stray inductances of the motor for the step-up chopper, a step-up chopper operation is performed only in the phases in which all windings of the electric motor are short-circuited while the commutation is being controlled by the inverter. In the remaining phases of the commutation control, the torque generation is controlled in the ordinary manner by space vector modulation to generate a rotary field.
A control system according to the invention first of all consists of the ordinary components of an EC controller, specifically a network rectifier and an inverter connected downstream via a DC link that is used to generate quasi-sinusoidal motor currents for a corresponding voltage clocking (modulation) of a PWM controller. According to the invention, the control system has an integrated step-up chopper with a controller designed in such a manner that, in terms of the method according to the present invention, a pulsating DC voltage rectified by the network rectifier is dynamically increased relative to its instantaneous values in such a manner that a resulting DC link voltage with reduced ripple always lies above a defined limit voltage over time. The step-up chopper has an inductor in series with a freewheeling diode and a downstream DC link capacitor, wherein the inductor is clocked with a pulsating DC voltage via a controllable electronic switch. The electronic switch for clocking the inductor is driven with pulse width modulation by a voltage regulator with variable clock frequency, specifically as a function of the respective current DC link voltage and a predetermined limit voltage. The controller activates the step-up chopper only in the phases of commutation in which all windings of the electric motor are short-circuited. In that way, the stray inductances of the motor winding heads can be used as the inductor for the step-up chopper.
The stray winding head inductances used according to the invention for the step-up chopper inductance depend, with respect to the size of their effective inductance, on the design of the respective electric motor or its stator and its stator windings. In an advantageous refinement of the invention, however, the stray inductances of the motor winding heads can be influenced, specifically, reduced or increased, by additional measures with respect to their effective inductance. For this purpose, magnetically active means with higher or lower magnetic permeability can be provided at least on one axial side of the motor or stator in order to influence the actually existing air gap permeability and the magnetic flux leakage. Some concrete examples of these measures will be described in more detail below.
Identical parts, components and dimensions are always provided with identical reference numerals in the various figures of the drawing.